I cannot read the sed magic, but the result is clear. Your boot partition is /dev/mmcblk0p1, and the sed magic says the device is then /dev/mmcblk, which is wrong. The device is /dev/mmcblk0. So I think the command should be
But what I need is to resize the /dev/root to be able to install more packages/software. Can you please tell me what I really need to do to achieve this? I'm really a beginner in filesystems and partitions.
rather than resize the existing root partition on partition 1 you might want to reconsider creating a 2nd large partition on your remaining disk space and format using F2FS file system and extroot to the 2nd partition. I believe F2FS works better on nand flash than ext4 especially with regards to wear leveling and just leave the 1st partition as it is for booting. The only downside to this config is it's troublesome when updating firmware as you need to extroot after each firmware update.
gdisk /dev/mmcblk0
enter the following in gdisk:
n
w
y
the above should automatically create a 2nd partition using all remaining disk space assuming you only have one partition.
mkfs.f2fs /dev/mmcblk0p2 (assume mmcblk0p2 is 2nd partition)
or -
mkfs.f2fs -o 30 /dev/mmcblk0p2 (this provisions 30% for future bad nand)
open LUCI and go to mount points select your mmcblk0p2 and mount it to /
a message box will automatically appear. SSH into openwrt and follow the instructions. CAVEAT: the instructions assume SDA1 but your device is MMCBLK0P2 so enter each instruction separately and change SDA1 for MMCBLK0P2.